In current Code Division Multiple Access (CDMA) communication systems a Radio Link Protocol (RLP) is utilized for the link layer to transport data traffic between a mobile unit and infrastructure equipment. RLP is a Negative-Acknowledgment (NAK) based protocol in that the receiver does not acknowledge correctly-received RLP frames. In-order delivery is accomplished with the use of a sequence number (SEQ) on each frame. RLP maintains a counter for the sequence number of the next new data frame to send [SEQ(S)] and a counter for the sequence number of the next new data frame it expects to receive [SEQ(R)]. RLP requests the retransmission of RLP frames when a frame is received with a sequence number greater than the next expected sequence number (SEQ(R)). Retransmission is accomplished by sending a NAK to the transmitter identifying the sequence number of the frame not received. Prior to receiving the NAK'd frame, subsequently transmitted RLP frames continue to be received by the receiver.
FIG. 1 illustrates the current RLP NAK procedure. As shown frames 101 are transmitted by a transmitter over the air and are received by a receiver as frames 102. During over-the-air transmission, oftentimes data is lost and needs to be retransmitted to the receiver. This is illustrated in FIG. 1 as frame F3 being lost. Upon receiving frame F4, the receiver immediately realizes that frame F3 is missing and requests retransmission of F3 by sending a NAK to the transmitter. Upon reception of the NAK, the transmitter immediately retransmits F3 to the transmitter.
A problem arises when the last data frame transmitted in a bearer data burst (e.g. an upload or download) by the transmitter is not received. In this situation, the receiver will receive no subsequent frames so the receiver will not find out the last data frame was never received, unless/until another bearer frame is sent by the transmitter. More particularly, because RLP determines lost frames by a break in frame sequence number, the receiver needs to receive subsequently transmitted frames in order to determine if a frame has not been received.
In order to solve this problem, prior art systems transmit a predetermined number of idle frames after the last data frame is transmitted. The idle frames comprise a frame number equal to the next frame number in the sequence of data frames. This is illustrated in FIG. 2. As shown in FIG. 2, the last data frame sent (F4) is followed by a series of idle frames (I5) and when F3 is retransmitted, the series of idle frames also follow. When the receiver receives the idle frame I5, it knows that the data transmission has ended, and that F4 is the highest data frame transmitted to the receiver.
Although sending idle frames solves the above-mentioned problem, another problem arises in that the transmission of idle frames negatively impacts radio frequency (RF) and network capacity. In particular, because a CDMA system is a self-interfering system, all transmissions over the system result in added system interference, network capacity load, and in some cases Mobile Subscriber (MS) battery drain. Therefore, a need exists for a method and apparatus for data transmission that allows a receiver to know when a last data frame has not been received, yet reduces the amount of system interference and network capacity loading relative to prior-art systems.